Acoustic device



July 11, 1933. w. L. WOOLF ET AL ACOUSTIC DEVICE Filed Sept. 10, 1929 ml. B700 Cor/wel Patented July 11, 1933 FFE WILLIAM: L. \VOOLF AND LIONEL B. CORNWELL, OF NEW YORK, N. Y.

ACOUSTIC DEVICE Application filed September 10, 1929. Serial No. 391,652.

This invention relates to acoustic devices for the reproduction of sound from varying electric currents.

An object of the invention is to devise a sound reproducer or loud speaker of the horn type having high cfiiciency over a Wide range of frequencies.

A further object is to devise a novel form of diaphragm and a novel sound chamber structure for cooperation therewith for the eiiicient tra nsn'iissi on of sound waves from the diaphragm to the mouth of a sound project-or or horn.

A specific object of the invention is to devise a sound chamber structure for elimi nating or minimizing wave disturbances or cross currents at the center of the diaphragm where the sound passage from the periphery of the diaphragm converges into an outlet passage arranged at right angles to the plane of the diaphragm.

A further specific object is to devise a sound chamber and a diap'hragm therefor in which the sound passages formed therein are devoid of sharp angles orturns which reduce the efficiency of sound transmission.

Our invention is illustrated in the accompanying drawing in which:

Figure 1 is an elevational view partly in section of one embodiment of our invention;

Figure 2 is a plan view of the vibrating diaphragm employed in F igurel; and

Figure 3 is a side view of Figure 2.

Referring to Figure 1, the loud speaker Which we have devised is of the QlGCtIO-(Il namic type and comprises a cylindrical casing or pot A of magnetic material, such for example, as steel, containing an annular magnetizing winding B, and having an annular polepiece O secured to the periphery of its open end. i A center pole piece D is mounted upon a central core A projecting from the bottom of pot A. A voice coil E mounted upon a circular diaphragm F is suspended in the annular air gap formed between pole pieces C and D. The vibrating diaphragm F is clamped at its periphery be tween rings C1 and C2 by suitable screws C3 passing through the pole piece C and through the rings C1 and C2 into a cap member G surmounting the vibrating diaphragm F.

The details of the diaphragm F areillustrated in Figures 2 and 3. The central por- I tion H of the diaphragm is domed or convex in shape and is provided with a pronounced tip or protrusion H at its center. Surrouini'ing the central convex portion is an annular section K corrugated with curved corrugations in the manner shown in Figure 2. The corrugations in section K are preferably arcuate in shape, whereby-the annular section K is stiii'cned, and bendingof the diaphragm takes place only at the outer and inner peripheries of'tlie 'corrugated'se'ction. ihis structure avoids any ripples which would otherwise be produced in section Kif this area were not stiffened. The'drive coil E is supported upon a cylindei' ofvery thin in sulating material indicated atli, which in turn, is cemented at one end to thediaphrag'm F. We prefer to make driving coil support L of such diameter th at it will be attached to the diaphragm F oil a circle at which the pressure exerted on the diaphragm within the circle during operation is equal the p'res sure exerted on the diaphragm outside of the circle. In this manner the driving jcloil is attached at points at which there is mechanical balance, and there will be no tend 30 ency for the diaphragm to seesaw about the points of attachment. Outside of the corrugated section is a flat peripheral section M by which the diaphragm is clamped between clamping rings C1 and C2. The flat pe- 35 ripher'al section M is provided with a number of holes M1 for receiving clamping screws C3, and a pair of notches M2 to facilitate bringing out the leads'of driving coil E to two insulated binding posts N (only one shown) mounted upon cap G (see Fig. 1). The diaphragm is preferably stamped by dies from a very thinisheet of-v light metal, such as duralumin.

The internal. surface of cap G is shaped with respect to the outer surface of diaphragm F to provide a sound passage be ginning at the extreme peripheral edge of the unclam ped part of thediaphragmfand leading to the central outlet'passa'g'e in cap G in such manner as to give an exponential increase in area from the periphery of the diaphragm to the beginning of the outlet passage. The central tip or protrusion H serves to eliminate the small air pocket which would otherwise be formed in the outlet passage at the center of the diaphragm. Such air pockets have been found to absorb the energy of the high frequency notes and substantially impair the transmission in the upper range of the audible scale. The space between the convexed central portion of the diaphragm F and the center pole piece D is adjusted to produce the same pressure against the underside of the diaphragm as exists on the upper side of the diaphragm during operation. This equalization of pressures produces equal loading on each side of the diaphragm and is materially effective in eliminating distortion which would otherwise occur. In other words, the acoustic impedances of the sound chambers associated with the two sides of the diaphragm are substantially equal. The cap G is preferably die cast from white metal which presents a very smooth and slick surface for the sound passage and materially increases the efiiciency of transn'iission of the high frequency notes.

Operation of the invention is as follows: The magnetizing winding B is energized with direct current from any suitable source con nected to the two coil terminals 0 (only one shown). The driving coil E is energized from a source of varying electric current which is to be reproduced into sound vibrations. As is well understood by those skilled in the art, the interaction between the magnetic field set up by current flowing in the driving coil E and the magnetic field induced across the annular gap between poles C and D causes the diaphragm to vibrate in accordance with variations of the electric current supplied to the driving coil. The stiffened convex portion H of the diaphragm vibrates as a whole, that is, it vibrates with piston-like action. In order to permit such piston-like movement of the central portion H, the cor-- rugated peripheral portion K vibrates with a hinge-like action about its outer peripheral edge. There is also a hinge-like action between the central portion H and the corrugated section K at the boundary between these two sections. The corrugations in annular section K prevent the formation of ripples or waves in this part of the diaphragm. It will be understood that a horn or sound projector is attached to the cap G to project the sound waves in the desired direction.

From the foregoing it will be seen that by employing the novel diaphragm with the central convex portion projecting into the housing cap G we have devised an arrangement in which the sound passage begins at the periphery of the vibrating diaphragm and gradually converges into the outlet passage of the cap with an increasing area and without sharp turns or bends. it will be observed that the convex shape of the diaphragm not only serves to stiffen the central area to produce a piston-like action of this portion, but the particular shape of the diaphragm also makes possible the formation of the highly etficient sound passage for leading the vibrations from the peripheral edge of the diaphragm to the center and into the outlet passage without undue interference. The tip H serves to prevent cross-currents and interference in the air pocket which would be formed in front of the center of the diaphragm if the tip were not present. It will be understood that the tip may be formed with a sharper central point than shown in the drawing if desired.

It will be apparent to those skilled in the art that the details of construction may be varied in many ways Without departing from our invention in its broadest aspect. it will also be understood that the operation of our device is reversible, and it may he used as a transmitter for converting sound vibrations into electric current variations.

We claim:

1. A sound reproducer comprising a housing forming a sound chamber provided with an outlet passage, a diaphragm mounted in said chamber comprising a substantially flat peripheral portion, and a relatively stiff domed central portion projecting into said housing, the wall of said housing cooperating with said diaphragm to form a sound passage with increasing cross-sectional area leading from the periphery of said diaphragm to the throat of said outlet passage.

2. A sound reproducer comprising a housing forming a sound chamber provided with an outlet passage, a diaphragm mounted in said chamber comprising a substantially flat peripheral portion, and a relatively stiff domed central portion projecting into said housing, the wall of said housing cooperating with said diaphragm to form a sound passage with increasing cross-sectional area leading from the periphery of said diaphragm to the throat of said outlet passage, and driving connections attached to the domed central portion of the diaphragm on the concave side thereof.

3. In a sound reproducer, a vibrating diaphragm comprising a domed central portion surrounded by a flat peripheral portion, and driving connections for said diaphragm secured to said domed portion on the concave side thereof at points lying on a circle intermediate the center and the peripheral edge of said domed portion.

4. A sound reproducer comprising a housing forming asound chamber provided with an outlet passage, a diaphragm mounted in said chamber comprising a substantially flat peripheral portion, and a relatively stifli domed central portion projecting into said housing, the wall of said housin cooperating with said domed central portlon to form a sound passage leading from the periphery of said diaphragm to the throat of said outlet passage adjacent the center of said domed portion, said domed portion being provided with a central tip extending beyond the contour of said domed portion and projecting into said outlet passage to prevent Wave interference.

5. A sound reproducer comprising a vibrating diaphragm, a horn, and a sound chamber for associating said diaphragm with said horn comprising a casing, the Walls of which form sound passages leading from said diaphragm to the throat of said horn, said casing being formed of an integral casting of White metal whereby said sound passages Will efliciently transmit high frequency vibrations produced by said diaphragm.

6. A sound reproducer comprising a housing forming a sound chamber provided with an outlet passage, a diaphragm mounted in said chamber comprising a substantially flat peripheral portion, and a relatively stifl domed central portion projecting into said housing, and a cylindrical driving coil attached to said domed central portion on the concave side thereof, said coil being of such diameter that the forces developed on the portion of the diaphragm Within said coil during operation are equal to the forces developed on the diaphragm outside of said coil.

7. A sound reproducer comprising a vibrating diaphragm, a sound chamber on one side of said diaphragm provided With an outlet passage, and a sound chamber on the other side of said diaphragm having an acoustic impedance substantially equal to the impedance of the first sound chamber.

8. A sound reproducer comprising a vibrating diaphragm, a sound chamber on one side of said diaphragm provided With an outlet passage, and a substantially closed air chamber arranged on the other side of said diaphragm having an acoustic impedance substantially equal to the impedance of the first sound chamber.

9. In a sound reproducer, a vibrating diaphragm comprising a domed central portion surrounded by a flat peripheral portion, and a cylindrical driving coil secured to said domed portion on the concave side thereof at points lying on a. circle intermediate the center and the peripheral edge of said domed portion.

In Witness whereof, We aflix our signatures.

LIONEL B. CORNWELL. W. L. WOOLF. 

